Alan Miller of the Center for Global Change [Volcanic Hyperbole," letters, July 29] warns that "printing both sides of a [scientific] issue can be seriously misleading." In the argument about whether growing human sources—like CFCs—or non-growing natural sources—like, volcanoes and oceans—are responsible for ozone-unfriendly chlorine in the stratosphere, both extreme positions can also be wrong.

One side simply argues that the huge natural sources dominate, the other side counters by calculating that almost all natural chlorine would be rained out before it reaches the stratosphere. The dispute has indeed degenerated into "polemics," as Mr. Miller quoted me as saying. Although this suggests that I favor the CFC source, I do not accept either theory. We need empirical evidence from direct observations of stratospheric chlorine, not calculations.

The published data have been controversial. In 1987 an authoritative article by the Belgian scientist R. Zander indicated little chlorine growth and therefore little contribution from CFCs. But a 1991 paper by NASA scientist Curtis Rinsland shows an increasing trend, and therefore an important contribution from CFCs, as well as from natural sources.